Optical systems, such as profile lamps or projectors, are limited in output by Etendue E=A*Ω, as the gate through which light is emitted has a limited opening area A and the imaging optics only collect light from a limited solid angle Ω. For light sources the Etendue can be calculated in the same way, where A is the radiating area, and Ω is the solid angle it radiates into.
It is fundamentally only possible to effectively utilize light sources of same or less Etendue as the imaging optics in this kind of optical system. So if the source Etendue is a close match to the Etendue of the imaging system there are no gains in using multiple sources in order to increase the light output (intensity/lumen) as the Etendue of the light sources then will be larger than the Etendue of the imaging system and the imaging system is thus not capable of collecting the light.
However there is an exception to this when the sources are of different colors and have spectral compositions with only a little overlap in spectrum. Then it will be possible to combine the different sources (colors) by an arrangement of appropriately-designed dichroic band pass/band stop (reflecting) filters. This is a well-known principle from LCD projectors, where “color cubes” are used to combine red, green and blue into white. Such color cube system is illustrated in FIG. 1b. For an illumination system red green and blue can also be combined by use of single sheets of dichroic reflectors/filters, such as the illumination systems illustrated in illustrated in FIGS. 1a, 1c and 1d. 
One disadvantage of these known color combiner solutions is the inability to combine sources with overlapping spectrums efficiently. White phosphor converted LED's have the highest efficacy of currently available LEDs and are therefore crucial for an efficient color mixing luminaire. Furthermore, the broad spectrum emitted by phosphor converted white and amber LED's can help improve the color rendering index of the light output compared to simple RGB LED solutions.
Another disadvantage of the known color combiners is the complexity of their production because three planes of sources and angled combiner filters require a lot of space.
U.S. Pat. No. 7,239,449 discloses an illumination module for color display, preferably for use in data or video projectors as well as rear projection television sets, in which the light from at least three luminescent diodes (LEDs) or LED arrays of the base colors red, green and blue is collimated at a point provided for connection to a display unit and ranged on an optical axis of the illumination module. An LED or an LED array of a base color with a beam path (Lr) oriented in the direction of the display unit is arranged on the optical axis of the illumination module. For the purpose of color mixing, the LEDs and LED arrays of the other base colors are attached in such a way that their beam paths (Lg, Lb) are laterally input in sequence under input angles (alpha, beta) of <90 degrees into the beam path (Lr) of the first base color.
WO2008/072197 discloses for instance a color cube system similar to the one illustrated in FIG. 1b. JP2006-139044 discloses both a color cube system similar to the one illustrated in FIG. 1b and a color-combining system using successive single sheets of dichroic reflectors/filters similar to systems illustrated in FIGS. 1a and 1c. WO2006/054969 illustrates a moving head light fixture based on a color cube system like the one illustrated in FIG. 1b and another moving head light fixture based on successive single sheets of dichroic reflectors/filters similar to systems illustrated in FIGS. 1a and 1c. 